Solvent Effects on the Structure of Petroleum Asphaltenes

Neutron
total scattering was employed to study the impact of a
solvent on the behavior of asphaltenes by comparing structural interpretations
of solid phase samples to those dispersed in 1-methylnaphthalene (1-MN)
as a function of temperature and asphaltene solubility. This powerful
technique enables structural interpretation on length scales ranging
from local molecular order to the formation of nanoaggregates and/or
fractal clusters in a single experiment. We observed that more unstable
asphaltenes exhibit greater aromatic stacking on the local molecular
length scale and also produce more frequent nanometer-sized associations.
However, when the temperature was elevated, the breakup of larger
asphaltene clusters was correlated to a reduction in the extent of
asphaltene side-chain interactions. Additionally, dispersed asphaltenes
demonstrated an increase in aromatic stacking as the temperature was
increased, suggesting an entropic driving force for assembly driven
by solvent depletion interactions. This observation is novel in asphaltene
science and serves as a pathway to assess the impact of local solvent
composition on both enthalpic and entropic assembly pathways. The
results suggest that while more unstable asphaltenes exhibit greater
aromatic stacking, further association may be assisted by an increase
in the disordered side-chain interactions. The experimental results
also serve as a useful benchmark to validate simulation predictions
of asphaltene structure.